"tkinter.dnd" --- Drag and drop support
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**ソースコード:** Lib/tkinter/dnd.py

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注釈:

  This is experimental and due to be deprecated when it is replaced
  with the Tk DND.

The "tkinter.dnd" module provides drag-and-drop support for objects
within a single application, within the same window or between
windows. To enable an object to be dragged, you must create an event
binding for it that starts the drag-and-drop process. Typically, you
bind a ButtonPress event to a callback function that you write (see バ
インドとイベント). The function should call "dnd_start()", where
'source' is the object to be dragged, and 'event' is the event that
invoked the call (the argument to your callback function).

Selection of a target object occurs as follows:

1. Top-down search of area under mouse for target widget

   * Target widget should have a callable *dnd_accept* attribute

   * If *dnd_accept* is not present or returns None, search moves to
     parent widget

   * If no target widget is found, then the target object is None

2. Call to *<old_target>.dnd_leave(source, event)*

3. Call to *<new_target>.dnd_enter(source, event)*

4. Call to *<target>.dnd_commit(source, event)* to notify of drop

5. Call to *<source>.dnd_end(target, event)* to signal end of drag-
   and-drop

class tkinter.dnd.DndHandler(source, event)

   The *DndHandler* class handles drag-and-drop events tracking Motion
   and ButtonRelease events on the root of the event widget.

   cancel(event=None)

      Cancel the drag-and-drop process.

   finish(event, commit=0)

      Execute end of drag-and-drop functions.

   on_motion(event)

      Inspect area below mouse for target objects while drag is
      performed.

   on_release(event)

      Signal end of drag when the release pattern is triggered.

tkinter.dnd.dnd_start(source, event)

   Factory function for drag-and-drop process.

参考: バインドとイベント
